This invention relates generally to airfoils and, more particularly, to turbine airfoils with parted spars.
Turbine airfoils include a blade tip, a blade length, and a blade root. Typically, a cooling system supplies pressurized air internally to the airfoil blade. The internal pressures created by the cooling system generate ballooning stresses at an outer skin of the airfoil blade. To prevent the internal pressures from damaging the airfoil blade, typically the outer skin is supported with a rigid spar which extends along the length of the airfoil.
External surfaces of turbine airfoils are subjected to high temperature gas flows during operation. Cooling a turbine airfoil prolongs the turbine airfoil useful life and improves turbine airfoil performance. Increasing the turbine airfoil performance enhances efficiency and performance of an associated turbine engine. As engine performance is enhanced, turbine airfoils are subjected to increased aerodynamic loading and higher temperature gas flows. To withstand such loads and temperatures, turbine airfoils may be fabricated using composite materials. Although such composite materials can withstand the loads and high temperatures, such materials usually are not as resistive to high temperature gradients as other known materials.
During operation, turbine airfoils are cooled internally with a pressurized cooling system. Accordingly, continuous spars operate at temperatures which are substantially less than the operating temperatures of the turbine airfoil outer skin surfaces. A temperature gradient between the continuous spar and the outer skin surfaces creates opposing thermal strains in both the continuous spar and the outer skin surfaces. The thermal strain mismatch created by the temperature gradient causes the continuous spar operating at a lower temperature to be in tension, and the outer skin surfaces to be in compression. Composite materials, such as ceramics, maintain a high modulus of elasticity and a low ductility at high temperatures, and the thermal stresses may cause cracks to develop within the continuous spars leading to failure of the turbine airfoil.
In an exemplary embodiment, a turbine airfoil includes a parted spar arrangement which reduces thermal stresses within the turbine airfoil. The turbine airfoil includes a blade tip, a blade root, and a blade span extending between the blade tip and the blade root. The blade span includes a skin covering extending over the blade span, and at least one spar arrangement having a length less than a length of the blade span and positioned between the blade root and the blade tip. The spar arrangement includes a plurality of spars including at least a first spar having a first side and a second side.
During operation, the turbine airfoil is cooled internally such that an outer skin covering surface operates at higher temperatures than that of the parted spar arrangement and temperature gradients develop between the parted spars and the outer skin covering surface. Because the airfoil uses parted spar arrangements, the turbine airfoil skin surfaces are permitted to thermally expand between parted spar arrangements which prevents thermal stresses from developing as a result of the outer skin surfaces operating at higher temperatures. Accordingly, the outer skin coverings and the parted spar arrangements are not subjected to the potentially damaging thermal strains of known turbine airfoils and may be fabricated from low strength and low ductility materials to provide a turbine airfoil which includes a spar arrangement that is reliable and cost-effective.